In the past, in order to realize reduction of power consumption, speed up of printing, enlargement of applicable types of paper, attention has been drawn to a toner for developing an electrostatic image (hereinafter also simply referred to as a toner) which is excellent in low-temperature fixability for fixing a toner image on a recording medium such as paper at a lower temperature than before.
As a means for improving the low-temperature fixability of the toner, a method in which a crystalline substance is contained in a binder resin may be mentioned. When a crystalline substance is contained, the volume resistivity of the toner lowers and the changeability of the toner tends to deteriorate.
Toward this problem, a technique has been proposed in which a specific structure is introduced into the toner to increase the volume resistivity so as not to impair the chargeability of the toner even if a crystalline resin is contained (for example, refer to Patent document 1: JP-A 2007-86494).
Here, in the production print area, there are many cases where the toner adhesion amount is large and an image over the entire paper is output to both sides of the paper. The problem that documents are stuck to each other electrostatically at the time of paper ejection has become obvious. This is caused as follows. When outputting images on both sides, due to the transfer current at the time of transferring the toner image (hereinafter also referred to as the back side image) to the second side (back side) of the paper, the charge is accumulated in an image (hereinafter, also referred to as a surface image) fixed on the first side (front side) of the paper. Normally, when a rear side image on a sheet is fixed, the surface image and the pressure roller of the fixing device come into contact with each other, and the charge leaks. When sufficient heat is not applied to the surface image during fixing, the electric charge remains on the surface image.
According to the technique described in Patent Document 1, since the volume resistivity of the toner at the time of heating is high, electric charge tends to accumulate in the surface image, and there is a problem that adhesion between the sheets easily occurs.